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4WD and AWD systems explained



  • rshollandrsholland Posts: 19,788
    Maybe in the serial #? They often denote what drivetrain the vehicle has. You may have to go to the dealer to get it translated, unless it's somewhere in the owner's manual?

    Also, this site focuses on Chrysler products. Maybe they can help.

  • almost all the information you will ever want to know about AWD and 4WD systems in small SUV/sedan platforms

    cliff notes

    Honda CRV--- basically FWD, 4WD is just badge
    system relies on loss of traction to give some power to rear wheels and as you can see it gives very little so you would be better of with FWD vehicle with LSD than this

    Toyota Highlander --- no better than CRV, i suspect Rav-4 to have same/similar system
    about same process as CRV

    Subaru, symmetrical AWD all wheels get same amount of power/torque all the time

    VW - as you can see is also really good but not the best

    these vehicles were tested because manufacturers claimed full time 4WD or AWD.

    Acura SH-AWD is 3.5WD system where front get ~70% of power until slip is detected.

    Infiniti ATTESA - all models that have "x" in the end of the model name. "part time AWD" when driving only rear wheels get power, if torque loss at the wheel is detected (no slip yet) and sends power to front also. capable of up to 50:50 distribution. this is a very short version as the system is very complicated and probably the best you can get for street performance. Inifiniti vehicles are not intended to be used off road

    Nissan Intelligent AWD, Rogue for example provides 50-50 power split between front and rear wheels when accelerating from a stop. As the vehicle gets to cruising speed, that basis will revert back to full front wheel drive . I-AWD predicts potential problems and automatically transfers to a 70-30 bias when taking moderate to sharp turns. The system still reacts to traction loss and sends power where needed, but it’s improved over the standard brainless AWD systems. Now rather than waiting until a traction loss and adapting as quickly as possible, the vehicle is already prepared by having that power bias already set
  • ateixeiraateixeira Posts: 72,587
    FWIW Toyota has been installing an upgraded system and it's trickling in to more models. The 2011 AWD Sienna's system is better than the 2010's, for instance.

    Cool vid, though. Even the most basic Forester is able to send enough power to either axle to climb that ramp.
  • kdshapirokdshapiro Posts: 5,751
    Subaru, symmetrical AWD all wheels get same amount of power/torque all the time

    This has been the topic of hundreds of posts. Bottom line, the statement as a generalization is not true, but exactly what happens under the covers can't seem to agreed on. At least reference STI with the VCCD? Probably don't have the acronym correct.
  • on the first page about the 4WD/AWD classification, should we have one more type called auto-AWD or part-time AWD ? During normal condition it is 2WD (mostly front), it transfers power to the other axle only after slippery. It is different from full-time AWD since full-time AWD at least has 5-10% power to the other axle. Examples are CR-V, Pilot, RAV4,...
  • Most Truck are part-time 4WD. 4W H or 4W L can not be used in dry/wet pavement.

    We got lots of snow in New England area. But they put sand/salt pretty fast on highway, so during/after snow, it is wet or with little bit snow or ice. How do we drive a truck on this road? Do we use 2WD, or 4WD H? If 2WD, then what is the advantage of truck. I saw lots of truck on highway that are running fast during/after snow. While small cars with 2WD just follow the traffic slowly.
  • I have a 2004 suburban 1500 LS 4WD. I bought it with 40K & had to have the transfer case rebuilt at 54K. The 4WD has been used maybe 10 times in 4H on snow only & never driven above 40-45mph. The vehicle now has 113K & I was going up the mountain & had it in 4H when I heard some metal on metal noise & a loud clunk come from either underneath or the front. I took it out of 4H & left it out. Put it in 4H while stopped & drove it 25ft with same metal noise. Ran in 2WD until got home when I tried to put it back into 4H and now the electronic switch defaults to AWD & won't actually engage the front. Put it up on racks, transfer case was a little low on fluid but no metal shaving or debris. When i try to change it to anything other than 2WD the electronic switch just blinks then defaults to AWD & front will not spin. The transfer case was serviced as scheduled at 90K. My question: Is there a common problem with this year of suburban & the 4WD or is it limited to mine. Rebuilding the transfer case twice seems ridiculous considering the minimal light use. Any thoughts?
  • wwestwwest Posts: 10,706
    "..Subaru, symmetrical AWD all wheels get the same amount of power/torque all the time..."

    I sincerely doubt that to be a TRUE statement. No automative design engineer, given a choice, would wish to allow a high level of torque to remain on the front wheels in a turn, tight turn, or even an accelerating turn, certainly not a tight accelerating turn.

    The tighter the turn or the harder acceleration being used in a turn the lower the torque delivered to the front should be.

    "..I-AWD predicts potential problems and automatically transfers to a 70-30 bias when making moderate to sharp turns..."

    I-AWD = Intelligent AWD...??


    Simply PRE-EMPTIVE, NOT inteligent by any means.

    An IDEAL, "INTELLIGENT" AWD system would incrementally reduce the engine torque to the front wheels as the turn becomes tighter and/or the acceleration level increases, all the way to 0-100 if need be.

    Since that cannot be easily done with ANY base FWD vehicle the best that can be done to help alleviate, reduce, the potential for loss of directional control in these instances is to partially REDUCE the level of engine drive torque to the front wheels.

    That's where the advantage goes, DRAMATICALLY so, to the SH-AWD system. The rear "diff'l" is over-driven by 7%(?) so the F/R torque distribution can be as high as 30/70.
  • wwestwwest Posts: 10,706
    "..if torque loss at the wheel is detected (no slip yet).."

    To my knowledge, limited as well as it might be, no such system is available. Torque loss at the wheel is ALWAYS the result of wheelspin/slip.

    That's why we now have so many PRE-EMPTIVE F/awd systems in the marketplace today. There is as yet no method of predicting that wheelspin/slip will occur as a result of application of too much engine torque for conditions. So manufacturers are choosing the alternative, most especially F/awd vehicle manufacturers.

    The alternative is to PRE-EMPTIVELY remove engine drive torque away from the front wheels at the times when wheelspin/slip is most likely to occur. That is: During acceleration from a stop, low speed acceleration, or when turning wherein directional control is often threatened due to wheelspin/slip. In a tight turn or accelerating turn due to the buildup of centrifical forces much of the front tire's roadbed cohesion will needed for lateral stability, directional control.

    Today's FWD vehicles, and in the past many F/awd vehicles, compensated for this problem via quickly dethrottling the engine should the VSC's yaw detection and stearing wheel position indicate that the front tires's traction coefficient would/might be exceeded.

    But there is a serious short-coming, design flaw, in these new pre-emptive F/awd systems. They engage the rear drive capability, in effect locking the center "diff'l", even on the roadbed surfaces with the highest tractive coefficient possible.

    That, of course, except when by pure happenstance the roadbed actually is slippery, results in a dramatically serious level of driveline windup and/or tire scrubbing. Stress and HEATING of the driveline components and early tire tread wear out/off.

    Look at the history of these new pre-emptive F/awd systems, the Ford Escape being one of the earlier adopters. "Pre-mature" PTO and rear diff'l drive clutch failures are/were the rule of the day. And the Acura MDX VTM-4 system, transaxle lockup failures as a result.

    The rumor is that the new 2011 Ford Explorer's F/awd system (no, that isn't a typo the new Explorer is a base FWD vehicle) uses water cooling for the PTO and the rear diff'l in an effort to abate the premature failures of these components resulting from the coupling of front and rear wheels even on high tractive conditions.

    Why not have a simple switch that the driver can use that enables the rear driving torque ONLY in known slippery conditions...?
  • wwestwwest Posts: 10,706
    Pine, "Soft", wood 1X2's inserted through all four wheels in such a way as to prevent wheel rotation absent breaking the 1X2's. My F/awd 2001 RX300 would break the front 1x2's with the slightest application of gas with the rear 1x2's remaining completely intact.
  • wwestwwest Posts: 10,706
    You'd be amazing at how quickly 4WD transfer case gearing will fail with even the slightest use on roadbeds that are intermittently slippery and tractive. My advice would be to NEVER make continuous use of the 4WD system except on SOLIDLY slippery surfaces and ONLY at relatively low road speeds.
  • steverstever Posts: 52,683
    Missed that one a couple of years ago eh?

    I thought about you yesterday. Did a U-Turn on a stretch of dead end pavement here in the UP of Michigan where we live now and got the Subaru stuck. :D

    Went a bit too far on the shoulder that's used for a snowmobile trail and the snow was soft and a bit wet. Had to shovel a yard away from behind each wheel (shod with "performance" all season tires btw).

    If I'd been in the safer FWD minivan, I would have done a 3 point turn and not had any problems. ;)
  • rshollandrsholland Posts: 19,788
    Steve, you're now a "Yooper?" My mother's side of the family is from Ironwood. Is that near you?

  • steverstever Posts: 52,683
    No kidding? That's fun, you betcha.

    Nothing is near us, but we have made it over to Ironwood.
  • I got a 2k bravada from my brother in law, the awd seemed to work great, but i got stock yesterday, and the front wheels didnt turn at all. instead of messing with a bunch of stuff to try and get it working, if there a way I can put a lever on it and make it manually controlled? Is this possible and has anyone done it before? Its my wifes car, and I want her to be safe. Let me know, thanks!
  • volkovvolkov Posts: 1,306
    Put a lever on what? Modern awd systems are electronically controlled. If it was stuck but the front wheels wouldn't turn, it is might be the traction control being too sensitive - it will brake the wheels that are slipping, and can take it to the point of not moving at all. Most vehicles have a button to turn the traction control/stability control off, and it can be useful in that situation. Look on the dash for a button with a picture of a car and S shaped tire tracks underneath. Not sure that the 2k had that yet though.
    Do the front wheels show power ever? If the rear wheels were slipping and the front weren't turning in AWD mode, there is a significant problem - abs module or centre diffy is toast would be most likely.
  • wwestwwest Posts: 10,706
    The facts of life are that other than the SH-AWD system(***) there really is NO such thing as a fully functional F/awd system in the marketplace today.

    As has already been said, many of these wannabee F/awd systems allow for TC to be turned off, leaving you with a simple ONEWHEEL drive system. That at least allows you to use wheelspin, if that works, to get unstuck or initially in motion.

    Your brother-in-law shows good sense, not risking his life in one of these. What did he get to replace..? I'm betting on a R/awd vehicle.

    *** Not anywhere near the equal of ANY R/awd system, such as a 4runner, but undoubtedly the best of the best insofar as vehicles with sideways mounted engines are concerned.
  • ateixeiraateixeira Posts: 72,587
    I like the ramp one:

    Ford Escape does terribly in that avoidance manuever.

    Results certified by USAC, too.
  • wwestwwest Posts: 10,706
    That particular ramp test more indicates an adequate rear LSD, maybe even a virtual one. A good driver could use slight, judicious, e-brake application in all cases to pull that ramp.

    Is it possible that's what the Subbie driver did?

    A rear LSD in a F/awd vehicle, SUBBIE...?? Rear LSDs are pretty rare, VERY rare, in F/awd vehicles.

    Oh, I wonder...

    A virtual LSD via use of TC differential braking at the REAR could be a possibility. Anyone know for sure..??

    So the Subaru doesn't have unequal front drive halfshafts thus "symmetrical" AWD...interesting
  • wwestwwest Posts: 10,706
    edited May 2011
    Driver trick, SIMPLE driver trick, trickery...??

    Look at the "set" of the Subbie's suspension at 1:48.

    Now notice the missing frames, video "glip" at about 1:45-7.

    During the frames you do not see the driver has quickly and very briefly oversteared to cause the rear wheels to "skid" out into the "line" he wishes to enter that upcoming hard left turn.

    Just as the Subbie driver is clearing the first set of cones look at the "line" of the Subbie and then at the angle, lack thereof, of the front wheels. The driver has cranked in enough overstear just ahead of the cones that the rear has come about, SWUNG OUT, and now he has counter-steared to correct for the overstearing condition.
  • ateixeiraateixeira Posts: 72,587
    That model has traction control managing the rear axle, so virtual LSD, yes.

    I think the difference is that the others have AWD or traction control since they are usually front axle only. That means the rear axle doesn't really have the virtual LSD, since they provide forward propulsion only part of the time anyway.

    USAC certifies the results, so presumably they didn't cheat with the handbrake...
  • ateixeiraateixeira Posts: 72,587
    edited May 2011
    Are they missing frames, or is it just a low refresh rate/resolution? Seems like the latter to me.

    Oversteer is consistent with what I used to get with my 98 Forester, in the days before stability/traction control (I had open front/rear diffs). It would wag the tail, swing out the rear, then AWD would shift power to the front axle and it would pull me out of the skid.

    Looks like the tail does get loose first, but the stability control catches it quickly, so it completes the turn with pretty good control.

    Doesn't seem like a trick to me - in fact that's more or less what I expect it to do. Watch 1:48 to 1:52, the rear view is closer and smoother, so you see it better.
  • wwestwwest Posts: 10,706
    It appears to me that only the run at ~1:40 was done at the higher speeds equal to the competition.

    Most stability controls, more modern ones anyway, will not interfere if the driver seems to be intentionally invoking overstear. And note that as soon as the rear wheels are aligned in the direction desirable for entering that first turn the front wheels are straightened, turned straight ahead.

    "..Oversteer is consistent..."


    You don't, typically, encounter overstear, tail "wag", in a FWD or F/awd vehicle unless it is invoked intentionally. And once you get a measure, the measure you wish, of that intentional overstear you had better be well ready to crank in the correct level of counter-steer, "early" stability control.

    No obvious deception required, just put highly experienced RWD/R/awd drivers behind the wheel of the competition.

    Or use just one driver who has been "coached".
  • ateixeiraateixeira Posts: 72,587
    My 98 was not FWD-based. It defaulted to 50/50 split and had a viscous coupling between.

    It would predictably break traction on the rear axle first. Weight distribution was 55/45 so the lighter side first makes sense to me.
  • wwestwwest Posts: 10,706
    edited May 2011
    Sorry, marketing wins yet another one.

    Your '98 Subbie either has, 1, a SOLID coupling between the transaxle output and the front drive diff'l input, with a VC to serve as a "soft", rubber bandish, coupling to the rear. Or, 2, has a standard "open" center diff'l with a VC mounted, connected across/between the two diff'l outputs. the

    Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.

    In either case the mostly "flaccid" VC MUST allow a great deal of "give" between the front driveline rotation rate and the rear driveline rotation rate. That's to avoid driveline windup and/or tire scrubbing while driving on reasonably tractive surfaces, especially when turning.

    VC fluid formulation, rate of fluid expansion with heat, determine the "attack" rate of "tightening" of the torque coupling to the rear. Most VC's also have a controlled level of inert gas inside the VC container to delay the onset of the VC's torque coupling. No torque is coupled until the gas is compressed to ZILCH..!

    Most marques manufacturering F/awd vehicles, including Subaru, have now abandoned the use of VCs for use on F/awd vehicles. Instant TC activation upon detection of wheelspin/slip precludes having any substantive level of disparate F/R wheelspin occur thereby making a VC useless.
  • rshollandrsholland Posts: 19,788
    All manual tranny Subarus, except the STI, have a VC.

  • wwestwwest Posts: 10,706
    How, where...?

    And I thought I had read the VC was discontinued in the latest models.
  • rshollandrsholland Posts: 19,788
    Nope. As I stated all manual transmission-equipped Subarus, except the STI, still use a viscous coupling on the AWD. Here's a SOA press announcement on the 2011 WRX, and in it is mentioned the viscous coupling:

  • ateixeiraateixeira Posts: 72,587
    edited May 2011
    Or, 2, has a standard "open" center diff'l with a VC mounted, connected across/between the two diff'l outputs.

    Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.

    It's #2, except the VC itself acts in place of the center diff, and yes it's fluid-filled like you described. Rotational differences between the front and rear axle thicken the fluid and tighten the coupling temporarily.

    By the way, the VC only applies to manual transmission models, though it's used in several Subaru models. They call it "Continuous AWD" in the brochures.

    Models with an automatic use a different system - no VC. Subaru markets those as "Active AWD", which uses an Electronically Controlled Variable Transfer Clutch. On higher-end models, "Variable Torque Distribution" or VTD is used. Finally, the STI uses DCCD.

    VDC is their marketing term for their stability control system, though in the past it referred to an Outback model that used the VTD AWD system, further confusing matters.

    So really there are 4 separate systems. Symmetrical AWD is a generic term that refers to all 4 of those, though I guess they do have equal length half-shafts in common across the whole lineup, so I guess that's what they're referring to.
  • ateixeiraateixeira Posts: 72,587
    Bob - remember something - ALL WRX models come in a manual, that's why all WRXs minus the STI use a VC.

    But there is no automatic WRX...

    Auto-equipped Foresters, Imprezas, and Outback Sports all still use the variable transfer clutch.
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